When a nutrient enters into water, aerobic organisms consume dissolved oxygen as a result of the induced metabolic activity. Thus, the nutrient exerts a demand on dissolved oxygen availability, which is called biological oxygen demand (BOD). If the amount of organic material in the medium is very high, it can lead to a decrease in dissolved oxygen concentration. At low oxygen levels, aquatic environment promote the growth of anaerobic species.
Anaerobic metabolism is much slower than aerobic processes (typically more than one order of magnitude) have lower efficiency, and generates various intermediate organic compounds (e.g. organic acids, alcohols, methane). As a result of the lower rate of dissolved organic matter consumption, this will accumulate in the aquatic environment.
If dissolved oxygen is consumed faster than it can be replenished, water starts to deoxygenate. No strictly aerobic organism, from microorganisms to fish, will survive in said water. Thus, organic contaminants will accumulate and further establish anaerobic conditions, which generate malodorous substances (e.g. sulfides and volatile amines) and partially oxidized organic compounds.
In addition to bad smell, anaerobic conditions can raise human health issues, because many anaerobic bacteria are pathogenic (for instance, tetanus, and botulism). When the water contains dissolved sulfates, reducing anaerobic bacteria produce H2S (corrosive and poisonous).
The increase of the amount of nutrients required for life in a water body is called eutrophication. Eutrophication is defined as the process of nutrient enrichment in a water body. It is a natural phenomenon in the ageing process of ponds and lakes (eutrophic lakes). On the contrary, a young water body, poor in nutrients required for life, is called oligotrophic. The nutrient increase in the pond promotes a higher production of aquatic plants and animals. Said organic matter increase generates in its turn an increase of the organic content of sediments. Eutrophication can generate serious problems in superficial water bodies.
Photosynthesis implies the creation of organic matter from inorganic materials and therefore the production of large amounts of organic substances where there were only little amounts before. When algae/plants die, their components are transformed in organic nutrients that exert an oxygen demand.
During photosynthetic action, CO2 is readily consumed, thus producing a rise in pH, which can attain a value over 10. During the night, the inverse reaction occurs, consuming oxygen and generating CO2, with which pH tends to drop. Photosynthetic activity has a significant effect on the pH level of the water body, because it affects the reversible reaction.HCO3−+H+←—→CO2+H2O
Finally, the masses of algae deposited in the shore die and rotten, thus producing anaerobic conditions, which present health dangers (e.g. formation of Clostridium botulinum, a strictly anaerobic pathogen microorganism). On the other hand, aquatic plant ramifications retain organic solids that decompose, which exerts a concentrated oxygen demand.
Generally, nitrogen N and phosphorus P are the limiting factors. In microorganism growth, P is consumed as phosphate, while the major part of bacteria assimilate N under the form of NH3, and only some of them assimilate N as NO3−. Inversely, algae assimilate N as NO3− and very few use NH3. There are more bacteria able to use NO3− as oxygen source than as N source. According to the approximate stoichiometry of photosynthesis in algae, N:P ratio is in the order of 7:1. According to the Liebig minimum law, an N:P ratio much higher than 7 in a water body indicates that P is the limiting nutrient; on the other hand, an N:P ratio value much lower than 7 implies an N limitation. Some authors suggest that P and N concentrations higher than 0.015 and 0.3 mg/l, respectively, are enough to generate an excessive growth of algae in lake waters.
The main sources of organic N are proteins, amino acids and urea; on the other side, inorganic N is in the form of NH3, NO3−, NO2−. Ammonia is a characteristic product of organic matter decomposition, and it can be microbiologically oxidized to nitrites and nitrates by the action of nitrifying bacteria. These processes occur naturally in water and constitute a major contribution to the biological oxygen demand.
When artificial water bodies are formed, such as lakes or ponds, water quality deteriorates progressively. Depending on the nutrient contribution, it can be reached any state from equilibrium in which algae, aquatic plants, bacteria, insects and fish survive in stable condition to eutrophication processes in which the excessive contribution of nutrients produces a high proliferation of algae and aquatic plants. When these die, they are decomposed by bacteria in aerobic processes that consume the oxygen. When oxygen decreases, many organic remainders remain deposited in the bottom, thus increasing sediments and suffering processes that increase turbidity, bad smells are produced and the physicochemical and sanitary quality of the water is impaired, which reduces the possibilities of recreational use.
To mitigate these effects different techniques are used, such as aeration systems to increase oxygen levels, algaecides and herbicides to control the excessive proliferation of algae and aquatic plants, the use of biological filters to decrease nutrient contribution, fish and zooplancton to reduce algae, nutrient capture by means of chemicals, inoculation of bacteria to digest organic matter, colorants to improve the aesthetic appearance, mechanic removal of algae and aquatic plants, the use of dredges to decrease the amount of sediment, clarifying agents to decrease turbidity, etc.
The characteristics and quality of the water of these ponds are very different to those of swimming pools. In the first case, an ecological equilibrium between different species must be attained, while in the second case the objective is the removal of organisms and impurities. Therefore, very different turbidity, color and physicochemical characteristics standards are accepted.
To keep swimming pool water transparent and apt for bathing, filtration systems are used, mainly sand, diatomaceous earth and cartridge filtration systems. The entire water must be filtered every 4 to 12 hours, depending on the type of swimming pool.
In addition, organic matter oxidants, disinfectants, algaecides and eventually pH regulators and clarifiers must be used to keep aesthetic and sanitary conditions. Depending on each country's regulation, swimming pools are required to keep minimal disinfectant residual concentrations or permanent redox potential (ORP) levels between 650 mV and 750 mV.
The application of the swimming pool technology to large water bodies to obtain optimal water quality is not possible due to the high cost of the installations and the involved operative costs.
To illustrate this situation, we can recall that if the water body to be filtered is the one described below in the application example of 250,000 m3, complying with the minimal regulations of Chilean Swimming Pools (T=2 en NCh 209, example country for the application), 2,983 liters per second are required to be filtered, which corresponds to the water volume treated by a potable water plant for a city. An Olympic swimming pool has 2,500 m3 (50×25×2 m), which corresponds to 1% of the considered volume in the application example of this patent application.
The same is true when swimming pools chemicals are to be applied to these volumes. The water volume of the application example of this invention corresponds to 4,000 10-meters-long swimming pools.
The control of disinfectants in swimming pools and spas by means of the measurement of the (ORP) has been used for many years with good results. ROP measures the oxidizing power of the disinfectant or, in other words, its real concentration-independent chemical activity. Direct measurement of disinfectant concentration can lead to error, because the activity can be decreased depending on pH and the presence of contaminants, even at high concentrations. In fact, studies have demonstrated that bacterial life in water is more dependent on ROP than on oxidant concentration. To remove undesired microorganisms in swimming pools, normally a ROP value between 650 mV and 750 mV is permanently maintained (public swimming pool regulations in developed countries require more than 700 mV permanently) at a normal pH between 7.2 and 7.6. This is not possible with large water bodies due to the high implied costs.
The previously exposed facts make maintaining large water bodies (over 15,000 m3) using filtration and disinfection technologies similar to those of swimming pools for recreational use largely unviable.
Therefore, there are no large artificial ponds or dams with the aesthetic and sanitary characteristics of swimming pools or tropical seas that have clarity levels higher than 25 and even 40 meters.
The technical problem solved with the present invention is the achievement of these characteristics in large water bodies at low cost.